1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a photo mask and a method for manufacturing the same.
2. Background of the Related Art
A photolithography process for patterning a material film on a semiconductor substrate is one of a group of processes for manufacturing a semiconductor device. A photo mask is used in selectively exposing photoresist film in the photolithography process. The photo mask is formed by patterning an opaque layer on a transmitting substrate according to a desired pattern.
A conventional photo mask comprises a structure in which a chrome layer, which is a transmission preventing layer, is formed on a light transmitting substrate such as a quartz substrate. A chrome oxide layer for preventing reflection by the chrome layer may be further formed on the chrome layer.
As semiconductor devices become more highly integrated, their design rule is reduced. Hence, the wavelength of the light source used in an exposing process is also reduced. Thus, it is necessary to use KrF or ArF as a light source in order to realize a feature size of no more than about 150-250 nm. Accordingly, it is necessary to form finer photomask patterns. It is also important to secure a processing margin during the fabrication of the photo mask. To do this, a dry etching method for the process of manufacturing the photo mask must be used.
However, it is difficult to apply the dry etching method to patterning the chrome layer used for the conventional photo mask. This is because the chrome layer has a low etching selectivity with respect to the photoresist layer for the reaction gas used in the dry etching. Chemical amplitude resist (CAR) or E-beam resist (EBR) are mainly used for the photoresist layer. These resist materials have a low etching selectivity with respect to the chrome layer for the reaction gas. Therefore, a very thick photoresist film is required for patterning the chrome layer using the dry etching method. However, resolution is reduced by using the very thick photoresist film, which is not suitable for manufacturing a highly integrated semiconductor device.
In order to compensate for this, a wet etching method is used for patterning the chrome layer. The wet etching method can limit the minimization of the feature size due to a wall angle and an edge slope. Accordingly, it is very difficult to realize a uniform line width of no more than about 1 .mu.m on the overall mask. As a result, it is difficult to realize a high resolution. Furthermore, when the chrome layer is patterned by the wet etching method, efficiency is relatively lower than when the chrome layer is patterned by the dry etching method.
The thickness of the transmission preventing layer must be reduced in order to realize high resolution. It is possible to reduce the thickness of the photoresist film for patterning the transmission preventing layer by reducing the thickness of the transmission preventing layer. Therefore, it is possible to form a finer-sized transmission preventing layer pattern, thereby increasing the resolution of the photo mask.
It is necessary to use a material layer having a higher optical density (OD) characteristic or a higher extinction coefficient k than the chrome layer in order to form the transmission preventing layer to a reduced thickness. A high extinction coefficient k or a high OD characteristic means that it is possible to selectively block the light used for the exposing process by a layer having a smaller thickness.
Another drawback of the chrome layer used for the conventional photo mask is that it has a relatively low adhesion to the quartz substrate, and thus a pattern defect may occur during the process of patterning the chrome layer. When the pattern defect occurs, a process of repairing the pattern defect by a focused ion beam (FIB) repair or a laser repair must be performed. During such repair, the substrate may be damaged, thus generating a river bed or reducing the transmissivity of the substrate.
Substituting a molybdenum silicide (Mo--Si) material layer for the chrome layer, for example a MoSi.sub.2 layer, has been attempted. Y. Watakabe, et al., "High performance very large scale integrated photomask with a silicide film", J. Vac. Sci. Technol. B(4), pp. 841-844, 1986. See also, Akira Shigetomi et al., "Fabrication technologies for advanced 5.times. reticles for 16M-bit dynamic random access memory", J. Vac. Sci. Technol. B8(2), pp. 117-121. 1990. Patterning the Mo--Si layer by a dry etch method has also been attempted. C. Pierrat, et al., "Dry etched molybdenum silicide photomasks for submicron integrated circuit fabrication", J. Vac. Sci. Technol. B9(6), pp. 3132-3137, 1991. Forming an anti-reflective layer on the Mo--Si material layer has been attempted as well. Akira Chiba, "Antireflective MoSi photomasks", J. Vac. Sci. Technol. B10(16), pp. 2480-2485, 1992. Research into applying the Mo--Si material layer on the phase shift mask (PSM) has also been performed. Rik Jonckheere, "Molybdenum silicide based attenuated phase-shift masks", J. Vac. Sci. Technol. B12(6), pp. 3765-3772, 1994. Research into using an amorphous silicon layer has also been performed. Charles H. Fields, et al., "The Use of Amorphous Silicon for Deep UV Masks", SPIE Vol. 1927, pp. 727-735, 1993.
However, the Mo--Si material layer has a low electrical conductivity due to the fact that the Si atom has a low conductivity when contained in the Mo--Si material layer. Accordingly, an additional charge dissipation layer should be used when the photoresist film is exposed using an electron beam. Also, Mo easily dissolves in chemical solution such as H.sub.2 O.sub.2, used in a cleaning process after performing patterning. Therefore, after performing patterning, film morphology may deteriorate or a pattern may be poor.